Phylum Echinodermata includes familiar marine life such as sea stars, sea urchins, and sea cucumbers. The name Echinodermata is derived from the Greek words for “spiny skin,” referencing the characteristic appearance of many members. This diverse group of approximately 7,000 living species possesses a unique suite of anatomical and physiological traits. These features reveal how these animals have adapted to thrive in marine environments across the globe.
Body Plan and Pentaradial Symmetry
A defining characteristic of adult echinoderms is their secondary radial symmetry, specifically a five-part arrangement known as pentaradial symmetry. While the free-swimming larvae exhibit bilateral symmetry, the adult form undergoes a metamorphosis where body parts organize around a central axis in multiples of five. This means an adult sea star or sea urchin has five equal sections radiating from the center, lacking a distinct left and right side.
The consequence of this radial organization is that the body is adapted for a non-directional existence, interacting with the environment equally from all sides. This symmetry is evident in the arrangement of their internal organs, such as the major nerve cords and the extensions of their water vascular system. Even in sea cucumbers, which appear more cylindrical and almost bilaterally symmetrical externally, the five-part radial organization remains internally apparent.
The Water Vascular System and Locomotion
Echinoderms possess a unique hydraulic system known as the water vascular system, a network of fluid-filled canals derived from the body cavity. This complex system is used for locomotion, feeding, gas exchange, and waste elimination. Water enters the system through the madreporite, a sieve-like plate on the aboral (upper) surface, which regulates pressure and filters incoming seawater.
The water then flows through a stone canal down to a ring canal that encircles the mouth, from which five radial canals extend outward into the body sections. Attached to these radial canals are hundreds of tiny, flexible appendages called tube feet, or podia, each connected to a muscular bulb called an ampulla. The coordinated contraction of the ampullae forces water into the tube feet, causing them to extend and attach to a surface using a temporary suction cup.
The alternating contraction and relaxation of the ampullae and muscles within the tube feet allow the animal to slowly creep along surfaces, a process that relies entirely on hydrostatic pressure. Beyond movement, the tube feet are also crucial for feeding, such as when sea stars use them to pry open the shells of bivalve mollusks. Furthermore, the thin walls of the tube feet allow for the diffusion of oxygen and carbon dioxide, serving a respiratory function.
Internal Skeletal Structure (Ossicles)
The body of an echinoderm is supported by an internal skeleton, or endoskeleton, embedded within the dermis layer of the body wall. This structure is composed of numerous small, interlocking plates made of calcium carbonate, called ossicles. These ossicles feature a unique, sponge-like porous micro-structure known as stereom, which makes the skeleton light yet robust.
In organisms like sea urchins, the ossicles are tightly fused together to form a rigid, hollow shell known as a test, providing substantial protection. In contrast, the ossicles in sea stars are more loosely articulated, allowing for greater flexibility in their arms. The spines that give the phylum its name are extensions of these ossicles, often articulating with the main skeletal plates via a ball-and-socket joint for movement and defense.
Exclusively Marine and Regenerative Abilities
All echinoderms are strictly confined to marine habitats, with no known species capable of surviving in freshwater or brackish environments. This limitation is due to their unique osmotic regulation and dependence on the ionic composition of seawater for internal processes, including the proper function of the water vascular system. They are found in every ocean basin, from the intertidal zone to the deepest abyssal trenches.
Echinoderms also possess remarkable regenerative capabilities, allowing many species to regrow lost or damaged body parts with impressive speed and completeness. A sea star, for example, can often regenerate an entire lost arm, and in some cases, a single arm can regenerate a whole new body, provided a portion of the central disk is attached. Sea cucumbers take this further, often deliberately expelling their internal organs, a process called evisceration, when stressed or attacked, only to fully regenerate the lost organs within weeks or months.
This lack of a centralized control center, or cephalization, means the nervous system is organized as a nerve ring around the mouth with radial nerves extending into each body section. This simple, decentralized nervous system may contribute to their ability to survive and regenerate after extensive body damage. The combination of their unique hydraulic system, calcified endoskeleton, and regenerative power makes Echinodermata a distinct and successful phylum.